Chapter 23The Evolution of

Populations

Question?

Is the unit of evolution the individual or the population?

So what do we study?

Population Genetics

Modern Synthesis

Population

Species

Gene Pool

Microevolution

Hardy-Weinberg Theorem

Basic Equation

Expanded Equation

Genotypes

Example Calculation

Let’s look at a population where: A = red flowers a = white flowers

Starting Population

N = 500 Red = 480 (320 AA+ 160 Aa) White = 20 Total Genes = 2 x 500

= 1000

Dominant Allele

A = (320 x 2) + (160 x 1)

= 800

= 800/1000

A = 80%

Recessive Allele

a = (160 x 1) + (20 x 2)

= 200/1000

= .20

a = 20%

A and a in HW equation

Cross: Aa X Aa Result = AA + 2Aa + aa Remember: A = p, a = q

Substitute the values for A and a

p2 + 2pq + q2 = 1

(.8)2 + 2(.8)(.2) + (.2)2 = 1

.64 + .32 + .04 = 1

Dominant Allele

A = p2 + pq

= .64 + .16

= .80

= 80%

Recessive Allele

a = pq + q2

= .16 + .04

= .20

= 20%

Result

Importance of Hardy-Weinberg

Example

PKU Frequency

Dominant Allele

Expanded Equation

Final Results

Practice Problem

A fruit fly population has a gene with two alleles A1 & A2. 70% of the gametes produced in the population carry A1. What is the proportion of the population that are heterozygous?

Practice Problem

In a H-W population with two alleles, A & a, that are in equilibrium, the frequency of allele a is 0.7. What is the percentage of the population that is heterozygous for this allele?

AP Problems Using Hardy-Weinberg

Solve for q2 (% of total). Solve for q (equation). Solve for p (1- q). H-W is always on the national

AP Bio exam (but no calculators are allowed).

Hardy-Weinberg Assumptions

If H-W assumptions hold true:

Microevolution

Causes of Microevolution

Genetic Drift

By Chance

Bottleneck Effect

Result

Importance

Founder's Effect

Result

Importance

Gene Flow

Result

Mutations

Result

Nonrandom Mating

Causes

Result

Natural Selection

Comment

Result

Genetic Basis of Variation

Polymorphism

Examples

Garter Snakes Gaillardia

Human Example

Other examples

Quantitative Characters

Yarrow and Altitude

Sources of Genetic Variation

Preserving Genetic Variation

Example

Result

Comment

Fitness - Darwinian

Relative Fitness

Rate of Selection

Modes of Natural Selection

Stabilizing

Directional Selection

Diversifying

Comment

Sexual Mate selection

Result

Comments

Question

Does evolution result in perfect organisms?

Answer - No

Evolution is limited by historical constraints

Adaptations are often compromises.

Chance, Natural selection and the environment

all interact

Selection can only act on existing variations

Summary

Know the difference between a species and a population.

Know that the unit of evolution is the population and not the individual.

Summary

Know the H-W equations and how to use them in calculations.

Know the H-W assumptions and what happens if each is violated.

Summary

Identify various means to introduce genetic variation into populations.

Know the various types of natural selection.